1. Field of the Invention
The present invention relates to a wiring board prepreg manufacturing method including step of impregnating a raw material composition made of thermoset resin with a porous layer formed by a wet gelation process, and a wiring board prepreg which can be obtained by this method, and a using method thereof. Particularly they are useful to manufacturing a double surface wiring board, a multilayer wiring board and the like which requires laser via working.
2. Description of the Related Art
Conventionally, for the step of forming a base material layer, an insulating layer and the like of a wiring board to be used in electronic equipment, a prepreg, in which thermoset resin is impregnated with fiber fabric or polymeric unwoven fabric and is half cured, has been used. Normally the prepreg is laminated with copper foil and used for it. For example, the step of laminating and curing the laminated body on a lower wiring layer or the like by thermally pressing the laminated body, and the step of forming a wiring pattern from the copper foil are repeated, so that a multilayer structure in which wiring layers and insulating layers are stacked successively is formed. Moreover, a laminate where copper foil is laminated on both surfaces of the prepreg by thermal pressing is used for a core substrate of a multilayer wiring board and a double-surface wiring board.
As a method of conductively connecting these wiring layers or metal layers before the wiring pattern is formed, a method of charging electrically conductive paste into a via hole formed in an insulating layer and conductively connecting the metal layers is known. More concretely as shown in FIGS. 3(a) through 3(e), a resin film 13 is further laminated on a prepreg 10 laminated on copper foil 11, and after an opening 5 which reaches the copper foil 11 is formed by the laser via working, electrically conductive paste 6 is charged thereinto by screen printing or the like, and the resin film 13 is peeled so that a surface of the electrically conductive paste 6 has a convex shape, and copper foil 12 is laminated so as to press a convex portion 6a and is thermally pressed so that the copper foil layers are conductively connected. Here, when the resin film is not laminated nor peeled, the convex portion is not formed on the electrically conductive paste, and thus a contacting pressure between the electrically conductive paste and the copper foil is not sufficient, and durability and reliability of the conductive connection between the wiring layers are easily deteriorated.
Meanwhile, besides as a covering layer during the laser via working, due to the reasons that handing property in the wiring board manufacturing method is heightened and adhesion of impurities is prevented, there are a lot of occasions that the prepreg is covered with a resin film. As a method of laminating the resin film on the prepreg, after thermoset resin is impregnated with a reinforcing fiber sheet, the resin film is laminated by a laminator or the like.
However, in the above method of laminating the resin film on the prepreg, it is necessary to prevent interposition of air (air biting) at the time of lamination, and it is necessary to adhere the resin film by a moderate adhesive force in order to peel it in the later step. Therefore, an apparatus, control and the like for the lamination become complicated.
Meanwhile, as a reinforcing phase of the prepreg, there exists an attempt to use a porous film made of polysulfone or the like. In the case where such a porous film is deposited by the wet gelation process (wet phase separating process), there exists a method of applying a polymer solution to a base material sheet and soaking it in a gelation liquid, but the obtained porous film and base material sheet are normally peeled after gelation.
Therefore, it is an object of the present invention to provide a wiring board prepreg with a resin film in which a specified resin film is adhered at the time of forming a porous layer, so that it is not necessary to additionally laminate the resin film and the resin film can be preferably used as a covering material at the time of the laser via working, and a manufacturing method thereof.
In the case where the prepreg is manufactured, when the thermoset resin is impregnated with the porous film or polymeric unwoven fabric, resin is dissolved in a solvent so that varnish is prepared, and after the varnish is applied to and impregnated with the porous film, the solvent is generally vaporized by heating or the like and simultaneously half cured. However, when the varnish is applied directly to the porous film, the porous film is swelled due to the solvent contained in the varnish, or the porous film is deformed due to a stress generated at the time of the application. Particularly in the case where the resin film is laminated, it is found that wrinkles are generated on the surface and an uneven portion is easily generated also on the resin film. As a result, for example after the laser via working, when the electrically conductive paste is applied into the via hole by screen printing, there arises a problem that the screen printing becomes impossible.
Therefore, it is another object of the present invention to provide a wiring board prepreg manufacturing method in which when the thermoset resin is impregnated, an influence of a contained solvent and deformation due to a stress are reduced so that generation of wrinkles or the like can be prevented.
Meanwhile, in the method of further laminating the resin film on the prepreg and peeling it after charging the electrically conductive paste, there arises a problem that a number of steps increases and the cost rises by the resin film.
Therefore, it is another object of the present invention to provide a wiring board prepreg using method where durability and reliability of the conductive connection are maintained satisfactorily and a number of the steps and the costs of raw materials can be reduced.
When the inventors got a conception that a base material sheet at the time of forming a porous layer is directly utilizing and studied hard about a combination materials which satisfy physical properties required for a prepreg and has excellent adhesion at the time of manufacturing, they found out that the above objects can be achieved by a combination of a porous film made of aromatic polyamide and a polyester film or the like, and finished the present invention.
Namely, a wiring board prepreg manufacturing method of the present invention is characterized by including the steps of: depositing and adhering a porous layer made of aromatic polyamide or polyimide onto a heat resistant base material sheet according to a wet gelation process; and impregnating at least one portion of a raw material composition of thermoset resin with insides of pores of the porous layer adhered to the heat resistant base material sheet.
According to the manufacturing method of the present invention, when the porous layer made of aromatic polyamide or polyimide is formed by the wet gelation process, a polyester film or the like is used as a deposition base material. For this reason, as is clear from the results of the examples, since the porous layer and the polyester film have suitable adhesion property, only the raw material composition of the thermoset resin are impregnated with them so that the prepreg with a resin film can be manufactured. Therefore, it is not necessary to additionally laminate a resin film like in the conventional art, and as mentioned later, the resin film can be preferably used as a coating material at the time of laser via working.
Further, when the inventors studies hard about various methods of applying varnish to the porous film or the like and its substituting method, they found out that the above objects can be achieved by laminating and pressuring a solid coating film, which is obtained by applying the raw material liquid of the thermoset resin to the base material surface and drying it, onto the porous film or the like, and finished the present invention.
Namely, another wiring board prepreg manufacturing method of the present invention is characterized in that the step of impregnating the raw material composition of the thermoset resin includes the step of applying a raw material liquid of the thermoset resin containing a solvent to a surface of a base material, the step of removing the solvent from the applied raw material liquid so as to obtain a solid coating film, and the step of while heating the solid coating film, laminating and pressurizing solid coating film together with the base material onto the porous layer so as to impregnate at least one portion of the solid coating film.
According to this manufacturing method, as is clear from the results of the examples, when the thermoset resin is impregnated, an influence of a contained solvent and deformation due to a stress are reduced so that generation of wrinkles or the like can be prevented.
Further, when the inventors studied hard about a method of conductively connecting metal layers without laminating a resin film for forming a convex portion of an electrically conductive paste, they found out that the above objects can be achieved by forming the thermoset resin layer which is partially exposed on the surface side of the resin porous layer and impregnating the exposed portion and simultaneously consolidating the electrically conductive paste at the time of the conductive connection by means of thermal pressing, and finished the present invention.
Namely, a wiring board prepreg using method of the present invention includes the steps of: using a wiring board prepreg where a half cured (B-stage) thermoset resin is partially impregnated with a porous layer made of aromatic polyamide or polyimide adhered to a heat resistant base material sheet so as to be partially exposed on a surface, peeling the heat resistant base material sheet from the prepreg; and thermally pressing the peeled prepreg which is interposed between metal layers so as to form a laminate whose whole thickness is reduced.
According to the using method of the present invention, the thermoset resin layer is formed so that at least one portion is exposed on the surface side of the resin porous layer, and the laminate as well as the metal layers are thermally pressed so that the whole thickness is reduced, and the electrically conductive paste charged into the opening of the laminate is consolidated. For this reason, even if a resin film for forming a convex portion is not laminated nor peeled unlike the prior art, a sufficient contacting pressure can be obtained between the metal layers and the electrically conductive paste at the time of the conductive connection. As a result, durability and reliability of the conductive connection can be maintained satisfactorily, and a number of the steps and the costs of base materials can be reduced.